It was just after midnight on July 4 when at least two dozen US Delta Force commandos arrived on heavily armed Black Hawk helicopters in Akrishi, a small town near the eastern Syrian city of Raqqa on the bank of the Euphrates River.

Before they landed to search for American hostages including journalist James Foley, they destroyed a crucial target: anti-aircraft weapons at a jihadist base about 3 miles (5 km) southeast of the city, a stronghold of Islamic State militants seeking to build a monolithic Islamic state.

The above account and other details of the raid have emerged from witnesses who spoke with a member of a Syrian opposition activist group, who identified himself as Abu Ibrahim al Raqaoui. Raqaoui told the information to Reuters in an interview via Skype from inside Syria.

His group also posted witness accounts of the raid on Facebook soon after it took place. The posts, which were viewed by Reuters, have since been taken down.

"The raid happened just after midnight," Raqaoui said. "The helicopters first started destroying anti-aircraft weapons." Reuters could not verify the account.

The White House publicized details of the raid on Wednesday, a day after Islamic State jihadists posted a video showing Foley being beheaded. The White House said the commandos failed to find Foley or other hostages and that it was prompted to make the announcement after several US news organizations learned of the operation.

The US military incursion into the heart of Islamic State territory, made on US Independence Day, ended in disappointment when the soldiers found no prisoners.